Cone Beam CT

Question 1

What is CBCT?

Answer 1

a form of cross-sectional imaging suitable for assessing radio dense structures

Question 2

What type of radiation is used for CBCT?

Answer 2

• ionising radiation
  
  • x-ray

Question 3

What shape is the beam used for CBCT?

Answer 3

pyrimidal/conical

Question 4

What shape is the digital receptor used for CBCT?

Answer 4

square

Question 5

How should the head be positioned for CBCT?

Answer 5

• horizontal
  
  • Frankfort plane parallel with floor
• vertical
  
  • midsagittal place

Question 6

What are the benefits of CBCT over plain radiograph?

Answer 6

• no superimposition
• ability to view from any angle
• no magnification/distortion
• allows for volumetric reconstruction

Question 7

What are the downsides of CBCT over plain film radiography?

Answer 7

• increased radiation dose
• lower spatial resolution
• susceptible to artefacts
• equipment more expensive
• images more complicated to manipulate
• images more complicated to interpret
• requires additional training

Question 8

What are the main benefits of CBCT compared to conventional CT?

Answer 8

• lower radiation dose
• potential for sharper images
• cheaper
• smaller footprint

Question 9

What are the main benefits of conventional CT compared to CBCT?

Answer 9

• able to differentiate soft tissue better
• cleaner images
• larger field of view possible

Question 10

What are the most common uses for CBCT in dentistry?

Answer 10

• clarifying relationship between impacted mandibular third molar and inferior alveolar canal prior to intervention
  
  • plain radiograph indicated close proximity
• measuring alveolar bone dimensions to help plan implant placement
• visualising complex root canal morphology to aid endodontic treatment
• investigating external root resorption next to impacted teeth
  
  • if not clear on plain radiographs
• assessing large cystic jaw lesions and their involvement of important anatomical structures

Question 11

What are the 3 common orthogonal planes used for CBCT?

Answer 11

1. axial
2. sagittal
3. coronal

Question 12

What is 3D volume reconstruction used for?

Answer 12

• picturing the extent/shape of a disease
• informative teaching aid for patient

Question 13

What are the drawbacks of 3D volume reconstruction?

Answer 13

• modified reconstruction of data
  
  • can be misleading
  • particularly poor for thin bones

Question 14

What imaging factors and variables are set before the scan and why?

Answer 14

• alter the information obtained
• alter the patient dose
• considered on case-by-case basis
• field of view
• voxel size
• acquisition time

Question 15

What is the field of view in CBCT and what is the decision on size based on?

Answer 15

• the size of the captured volume of date
• decision based on clinical case
  • increased size
    
    • increased radiation dose
    • increased tissues irradiated
    • increased scatter
    • larger report required

Question 16

What is the voxel size in CBCT and what is the decision on size based on?

Answer 16

• the image resolution
  
  • voxels are 3D pixels
    
    • CBCT larger than plain radiographs
• decision based on clinical case
  
  • decreased voxel size increases dose
  • decreased voxel size increases scan time
• typical range of voxel sizes
  
  • 0.4mm3-0.085mm3

Question 17

What imaging factor choices are used for endodontic cases?

Answer 17

• field of view as small as possible
  
  • 5x5cm
  • unless case with large apical pathology
• smaller voxel size

Question 18

What imaging factor choices are used for implant planning cases?

Answer 18

• field of view depends on number and position of implants
• larger voxel size

Question 19

What is the radiation dose delivered from CBCT?

Answer 19

• cannot determine single generic value
  
  • equipment
  • field of view
  • position of field of view
  • voxel size
• 13-82uSv
  
  • studies show up to 674uSv

Question 20

What are artefacts?

Answer 20

• visualised structures on the scan that were not present in the object investigated
  
  • most can be avoided or reduced

Question 21

What are the two main types of artefact?

Answer 21

• movement artefacts
• streak artefacts

Question 22

What are movement artefacts, how do they appear and what can be done to reduce them?

Answer 22

• occur if patient is not completely still
  
  • affects whole scan
• general blurriness
• extra contours
• reduced with fixation aids
  
  • chin rest
  • head strap

Question 23

What are streak artefacts and what issues do they cause?

Answer 23

• caused by high attenuation objects
  
  • primary metals
    
    • amalgam
• main issues
  
  • preventing caries assessment
    
    • in adjacent restorations
  • preventing endodontic assessment
    
    • perforations
    • missed canals

Question 24

What contra-indications exist to CBCT?

Answer 24

• if plain radiographs are sufficient
• pathology requiring soft tissue visualisation
  
  • malignancy
  • infection spreading in soft tissue
• high risk of debilitating artefacts
  
  • post retained crowns
  • amalgam restorations
  • metal ceramic crowns
• patient factors
  
  • unable to stay still
    
    • Parkinson’s disease
    • certain learning difficulties
    • uncooperative child
  • patient unable to fit into machine
    
    • kyphotic patient
    • unfavourable neck to shoulder ratio
      
      • obese
      • bodybuilder

Question 25

What training is required to carry out CBCT?

Answer 25

• UK undergraduate training is inadequate
  
  • referring
  • justifying
  • interpreting
• postgraduate training is required
  
  • level 1
    
    • IRMER duty holders
  • level 2
    
    • justifying, performing or interpreting
  • refresher training required
    
    • every 5 years

Question 26

What is required for a CBCT to be justified?

Answer 26

• must be preceded by a clinical exam
• only used when plain radiograph is not sufficient